Electrical Impedance Tomography (EIT) is an imaging method that seeks to create cross-sectional maps of electrical resistivity or impedance distribution inside the body. This has previously been done using a 16 electrode array fixed to the external perimeter of a body about a transverse plane for example as described in U.S. Pat. No. 4,617,939 (Brown & Barber). The electrical current causes a change in the electrical potential on the surface of the body being examined. The other electrodes of the array are used to measure the electrical potential on the surface of the body and thereby monitor the electric field created by the current pattern. Distortions in the field pattern may be related to changes in the impedance distribution inside the body. As the solution of impedance distribution from surface voltage measurements is generally ill-posed, it has not been effective for producing good static images of body organs. This has limited the adoption of the technique for general use.
The EIT process may be contrasted with other bio-electrical procedures such as bioimpedance spectroscopy. Bioimpedance spectroscopy is a process whereby four electrodes are situated at standard reference points on the body (for example, right and left wrists, right and left ankles). The actual positioning of the electrodes could vary with application. The impedance measurements are made with this group of four. Two electrodes are nominated for current flow and the other two are used for measuring voltages. Impedance is measured as a function of frequency (say, over the range 1 kHz to 1 MHz) and the results may be displayed as an R vs. X (resistance vs. reactance) plot over this range or simply as the modulus |Z| or phase or some version of this. The impedance or R or X or related measure would be considered as a dependent variable of measures such as for example % water or % fat, sex, height, extent of bleeding (these being things that may be given a priori or solved for) in a standardized empirical function and so a given Z would be used to extract a parameter such as the extent of bleeding. The success of the process depends significantly on how good the empirical function is and how ‘standard’ the subject. Use of this procedure to detect intraperitoneal bleeding has had very limited success.
Serious injury to internal organs—for example, as can be suffered during blunt trauma associated with road accidents—is usually indicated by the presence of internal bleeding. It is the rate of the internal bleeding, in addition to the total amount of blood lost, which is indicative of serious injury and relative urgency of treatment. A rate of more than 30 ml per minute is usually an indication that intervention may be necessary. Bleeding is usually monitored by monitoring vital signs such as pulse rate, blood pressure and skin colour. However, this is not always a consistent way to detect serious internal bleeding—particularly among younger trauma victims.
The use of EIT for detecting bleeding was discussed in the paper: “Detection and Quantification of Intraperitoneal Fluid Using Electrical Impedance Tomography” by Rosalind J Sadleir and Richard A Fox, IEEE Transactions on Biomedical Engineering, Vol. 48, No. 4, April 2001, pages 484-491.
While EIT has shown considerable promise for detection of intraperitoneal bleeding and other uses, its use has been limited due to certain problems inherent in the technique as used to date. The problem solved by EIT is inherently non-linear which has limited the usefulness of images reconstructed according to linearized approximations. Additionally, the accuracy of the results is limited due to extraneous variations occurring during the test period. Chief amongst these is the effect of breathing. Impedance measurements are particularly sensitive to the changes in abdominal shape and lung air quantity during the breathing cycle. In addition, the electrodes previously used for obtaining EIT images of the abdomen have typically comprised a belt with 16 electrodes adapted to be positioned all around the perimeter of the abdomen. This can be problematic for practical use on patients, especially those where spinal injury is involved. Such belts have also been susceptible to pick up of electrical noise on voltage inputs.
Throughout this specification, the term “tissue” will be taken here to include fluids such as blood and lymphatic fluids as well as other types of tissue.
The above description of the prior art is given to assist the reader form an understanding of the nature of the invention disclosed herein. However, this description is not to be taken as indicating that the disclosure in that prior art in any way forms part of the common general knowledge in the art.